Article

Background

Phytoplankton produce approximately half of Earth’s total oxygen by converting carbon dioxide and nutrients into complex organic compounds and by releasing oxygen, a process known as photosynthesis. Surprisingly, the total biomass of phytoplankton is about one percent of plant biomass on land. Yet, during the spring bloom in the North Atlantic, these microscopic plant cells can be easily spotted from outer space. Regions of large phytoplankton blooms support healthy and productive marine ecosystems via the consumption and transport of phytoplankton and energy throughout the food web. The trophic imbalances between predator (zooplankton) and prey (phytoplankton) are examined in a recent paper published by Behrenfeld in the September issue of Nature Climate Change.

Figure 1. Satellite and microscopic images of phytoplankton assemblages. Photo on left by NASA. Photo on right by Richard Kirby, Plymouth University

Environmental conditions

Phytoplankton inhabit the upper sunlit layer of the ocean, called the euphotic zone. There are many key environmental resources that limit phytoplankton growth in the euphotic zone such as temperature, light and nutrients. These resources undergo changes depending on the depth of the seasonal mixed-layer, which is the depth that surface water is homogenized by mixing. The depth of the mixed-layer increases during the winter and decreases in the late spring and summer.

Behrenfeld used assimilated data in a physical ocean model to show the coupling between phytoplankton concentration and mixed-layer depth. When the mixed layer deepens in the fall, division rates of phytoplankton slow due to light limitation. Phytoplankton concentrations decrease during this time due to elevated grazing and mortality. Eventually, low phytoplankton concentrations reduce the abundance of grazers and the deep mixed-layer causes phytoplankton and grazers to be physically diluted (e.g. they do not encounter each other as often). During the winter, phytoplankton mortality due to grazers decreases beyond light limitations on phytoplankton cell division. The phytoplankton bloom is then initiated during the winter when phytoplankton biomass begins to increase, while concentrations remain the same due to the increased volume of the deep mixed layer. Phytoplankton concentrations only begin to increase once the depth of the winter mixed layer transitions from deepening to shoaling (approximately February). These findings suggest that phytoplankton loss rates are mechanistically coupled to division rates and ocean physics, and that these conditions are likely to undergo changes in a warming climate that may decrease the ocean mixed-layer depth at high latitudes and consequently decrease phytoplankton biomass.

Figure 2. Phytoplankton concentration (chlorophyll) and the mixed-layer depth in the North Atlantic during winter.

Further evidence shows that phytoplankton division rates are not proportional to biomass maxima. For instance, phytoplankton cell division in tropical oceans can be greater than at high latitudes, but the tropics are incapable of producing large blooms as observed in the subarctic. The decoupling between cell division rates and biomass suggests that phytoplankton stock must be controlled by ecological factors in addition to limiting environmental resources.

Predator-prey dynamics

The primary consumer of phytoplankton is zooplankton, which are small organisms that drift through the water column. Grazing by zooplankton is enhanced by higher encounter rates in shallow mixed-layer environments. Zooplankton grazing decreases phytoplankton biomass, as does viruses and sinking out of the mixed-layer. The main idea of Behrenfeld’s paper is that changes in phytoplankton biomass are closely coupled to their predators. This can be described as a trophic seesaw, in which phytoplankton is on one side of the seesaw, while zooplankton is on the other side. As long as phytoplankton division rates are accelerating, they will be ahead of their predators. Once phytoplankton division rates slow down, the predators will continue to catch-up. The tilt of the seesaw is largely controlled by these predator-prey imbalances, and is also influenced by limiting environmental conditions that perturb phytoplankton biomass stock.

Seesaw in time

The timing and magnitude of the annual phytoplankton bloom varies from year to year. Behrenfeld examines phytoplankton concentrations over ten years in the North Atlantic using ocean color estimates from satellites. Ocean color allows for the determination of how much chlorophyll is present in the ocean surface layer, which is a proxy for phytoplankton concentration.

Behrenfeld noticed that fourfold variations in phytoplankton concentration are a result of subtle deviations in accumulation rates (Figure 4b, red line). More importantly, the accumulation and division rates are similar. Phytoplankton division rates are primarily driven by physical changes in light availability and mixed-layer depth (Figure 3). These findings imply that phytoplankton biomass is largely dependent on increasing and decreasing cell division rates, which impact zooplankton-phytoplankton imbalances.

Significance

Physical ocean conditions that impact phytoplankton blooms, such as light penetration and mixed-layer depth, are likely to undergo climate-mediated changes. A warmer, more stratified ocean surface layer may reduce the ability for phytoplankton to accelerate cell division rates during blooms, which has potential consequences for zooplankton and other predators that rely on phytoplankton for energy. Phytoplankton are also important in the exchange of carbon dioxide from the atmosphere to ocean. Hence, fewer phytoplankton in a warmer climate could affect natural carbon cycling.

Related

Discussion

4 Responses to “Zooplankton versus Phytoplankton: a trophic seesaw”

This is a very important paper indeed, and Behrenfeld’s new perspective on phytoplankton blooms is indeed quite a paradigm shift.
However, I think there is a definite misunderstanding of Behrenfeld’s Disturbance Recovery Hypothesis. In particular, what Behrenfeld says is that due to decrease of phytoplankton biomass during mixed layer deepening, density-driven grazing losses (and encounter rates between predators and their prey) decrease to the point where division rates EXCEED loss rates, thus the bloom initiates (aka, the accumulation rate is positive) IN THE WINTER!!. What is counterintuitive is that there is no increase in phytoplankton concentration until mixed layer deepening stops.
I encourage you to revise your article accordingly.

Hi Francoise, thank you for your comment. You are indeed correct that the bloom initiates during the winter when low phytoplankton biomass and grazers are physically decoupled, and phytoplankton division rates begin to accelerate. I have re-examined Behrenfeld’s paper and clarified the article where needed. I appreciate your feedback.

A well written and important article. Lots of good science but don’t need an M.S. to understand it. Thank you for your work on this. Not nearly enough people understand how much oxygen the ocean produces.